Ferrite Number (FN) Controlled Stainless Steel Coil Tube — Technical Specification
1. Product Overview
The Ferrite Number (FN)-controlled Stainless Steel Coil Tube is a precision-engineered coiled tubing product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is distinguished by its rigorous metallurgical microstructure control, specifically governing the delta ferrite content within the weld metal and base material matrix. The Ferrite Number — a dimensionless index quantifying the volume fraction of delta ferrite (δ-ferrite) — serves as a critical metallurgical parameter in ensuring weldability, corrosion resistance, and mechanical integrity of stainless steel coil tubes.
By maintaining precisely calibrated FN values, this coil tube product delivers superior resistance to hot cracking in weld zones, optimized phase balance in duplex stainless steels, and enhanced resistance to intergranular corrosion and chloride stress corrosion cracking (Cl-SCC). The product is suitable for demanding applications in the oil and gas, chemical processing, nuclear, and marine industries where metallurgical reliability is paramount.
2. Material Composition and Metallurgical Properties
2.1 Ferrite Number Definition and Significance
The Ferrite Number (FN), also referred to as the Delta Ferrite Index, is a standardized measure of the amount of retained delta ferrite present in the solidification microstructure of stainless steel welds and castings. It is expressed as a numerical value on the Schaeffler or DeLong ferrite number charts, which correlate the Cr-Equivalent (Cr-Eq) and Ni-Equivalent (Ni-Eq) compositions with the resulting ferrite content.
The presence of controlled delta ferrite is essential for the following metallurgical reasons:
- Hot Cracking Resistance: Delta ferrite provides a lower-melting-point second phase that absorbs sulfur and phosphorus segregations, thereby mitigating solidification (hot) cracking in weld metal.
- Stress Corrosion Cracking (SCC) Mitigation: An appropriate ferrite fraction reduces susceptibility to chloride-induced stress corrosion cracking in austenitic stainless steels.
- Phase Balance in Duplex Steels: In duplex stainless steels, maintaining a near-equal balance of austenite (γ) and ferrite (δ) phases — typically 40–60% ferrite — ensures optimal mechanical properties, corrosion resistance, and toughness.
- Magnetic Properties: The ferrite content directly influences the magnetic permeability of the material, which is a critical parameter in non-destructive testing (NDT) applications.
2.2 Applicable Grades
This FN-controlled coil tube product is available in the following primary stainless steel grade categories:
- Austenitic Grades: 304, 304L, 316, 316L, 321, 347, 904L
- Duplex Grades: 2205 (UNS S31803/S32205), 2507 (UNS S32750/S32760), 253 SMO
- Super Duplex Grades: 2507, 253 SMO, UNS S32750
- Custom Alloy Grades: Available upon request with specified FN targets
3. Technical Specifications
3.1 Ferrite Number Control Parameters
| Parameter | Specification | Test Method | Acceptance Criteria |
|---|---|---|---|
| Ferrite Number (FN) — Austenitic Weld Metal | 3–10 FN | ASTM E1926 / Schaeffler Chart | ≥ 3 FN minimum for hot crack resistance |
| Ferrite Number (FN) — Duplex Steel Base Metal | 40–60% δ-ferrite phase fraction | ASTM E1926 / Magnetic Method | 45–55% target; 40–60% acceptable range |
| Cr-Equivalent (Cr-Eq) | C + Mn + 0.5×Mo + 3.3×Nb + 3.3×Ti + 1.5×Si + 1.5×Cu + 0.5×N + Cr | Composition analysis | Per grade specification |
| Ni-Equivalent (Ni-Eq) | Ni + 30×C + 0.5×N + 0.3×Cu + 0.5×Mn | Composition analysis | Per grade specification |
| FN Measurement Tolerance | ±3 FN (austenitic); ±5% phase fraction (duplex) | ASTM E1926 | Within ±3 FN of specified value |
| Microstructural Phase Distribution | Uniform; no banding or segregation | SEM / Optical Metallography | No localized ferrite-rich or austenite-rich zones |
3.2 Dimensional Specifications
| Dimension | Specification | Tolerance | Standard |
|---|---|---|---|
| Outer Diameter (OD) | 12.7 mm – 219.1 mm (½" – 8") | ±0.5% | ASTM A795 / ASTM A778 |
| Wall Thickness | 0.5 mm – 12.7 mm | ±10% or ±0.1 mm, whichever is greater | ASTM A795 / ASTM A778 |
| Coil ID | 200 mm – 600 mm | ±3 mm | Customer specification |
| Coil Length per Spool | Up to 1200 m (continuous) | As required | Customer specification |
| Straight Length (Optional) | Up to 6000 mm | ±5 mm | ASTM A795 |
3.3 Mechanical Properties
| Property | 304/316 (Austenitic) | 2205 (Duplex) | 2507 (Super Duplex) |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 550 MPa | ≥ 620 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 450 MPa | ≥ 550 MPa |
| Elongation (A50mm) | ≥ 40% | ≥ 25% | ≥ 25% |
| Hardness (HV) | ≤ 200 HV | ≤ 350 HV | ≤ 380 HV |
| Impact Energy (20°C) | ≥ 200 J | ≥ 275 J | ≥ 275 J |
3.4 Chemical Composition (Typical)
| Element | 304L (Austenitic) | 316L (Austenitic) | 2205 (Duplex) | 2507 (Super Duplex) |
|---|---|---|---|---|
| C (max) | 0.03% | 0.03% | 0.030% | 0.030% |
| Cr | 18.0–20.0% | 16.0–18.0% | 22.0–23.0% | 24.0–26.0% |
| Ni | 8.0–12.0% | 10.0–14.0% | 4.5–6.5% | 6.0–8.0% |
| Mo | — | 2.0–3.0% | 3.0–3.5% | 3.0–4.0% |
| N | — | — | 0.14–0.20% | 0.24–0.32% |
| Estimated FN (Base Metal) | 5–15 FN | 5–15 FN | 40–60% δ-ferrite | 40–60% δ-ferrite |
4. Ferrite Number Determination Methodology
4.1 Schaeffler / DeLong Ferrite Number Charts
The Schaeffler and DeLong ferrite number diagrams are graphical tools used to predict the equilibrium ferrite content in weld metal based on the Cr-Equivalent and Ni-Equivalent values derived from chemical composition. The DeLong chart is an updated and refined version of the original Schaeffler diagram, incorporating additional alloying elements and refined boundary curves.
Procedure:
- Determine the full chemical composition of the base metal or filler material via optical emission spectrometry (OES) or inductively coupled plasma (ICP) analysis.
- Calculate the Cr-Equivalent and Ni-Equivalent values using the standardized formulas.
- Plot the calculated values on the Schaeffler/DeLong chart to read the corresponding Ferrite Number.
- Verify the predicted FN against measured values using the magnetic method.
4.2 Magnetic Method (ASTM E1926)
The ASTM E1926 magnetic method is the industry-standard technique for directly measuring the Ferrite Number in stainless steel welds and base metal. This method exploits the ferromagnetic nature of delta ferrite in contrast to the paramagnetic/weakly ferromagnetic austenite phase.
Instrumentation: A calibrated FN meter (e.g., FerriteScope®) equipped with appropriate probes is used. The measurement principle involves inducing a magnetic field in the material and measuring the magnetic permeability, which is correlated to the ferrite fraction through a calibration curve.
Measurement Protocol:
- Surface preparation: Grind and polish the test area to remove oxide scale and surface contamination.
- Calibrate the FN meter using standard reference blocks of known FN values (typically 0 FN, 10 FN, 20 FN, 30 FN, and 40 FN).
- Take multiple readings (minimum 5) at different orientations on the prepared surface.
- Record the average FN value; individual readings shall not deviate more than ±5 FN from the mean.
- For duplex steels, convert FN readings to phase fraction using the relationship: δ-ferrite % ≈ FN × 2.5 (approximate conversion, grade-dependent).
4.3 Metallographic Verification
For critical applications, metallographic examination per ASTM E4 provides visual confirmation of phase distribution and fraction. Techniques include:
- Light Optical Microscopy (LOM): Using specific etchants (e.g., Murakami's reagent) to differentiate austenite and ferrite phases.
- Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS): For precise phase identification and quantitative area fraction analysis.
- X-ray Diffraction (XRD): For bulk phase fraction determination in duplex steels.
5. Manufacturing Process
5.1 Steelmaking and Casting
Production begins with vacuum arc remelting (VAR) or electro-slag remelting (ESR) of qualified steel billets, ensuring low impurity levels (S ≤ 0.015%, P ≤ 0.030%) and uniform composition. The chemical composition is carefully controlled to target the desired Cr-Eq and Ni-Eq values, thereby establishing the foundation for the specified Ferrite Number.
5.2 Hot Rolling
Remelted billets are reheated to 1100–1200°C and hot-rolled to the target coil tube dimensions. The rolling schedule — including temperature, reduction ratio, and cooling rate — is optimized to produce a homogeneous austenitic or duplex microstructure with the target ferrite content.
5.3 Solution Annealing and Pickling
Following hot rolling, the coil tube undergoes:
- Solution Annealing: Heating to 1050–1150°C (austenitic) or 1050–1100°C (duplex) in a controlled atmosphere (protective gas or vacuum) to dissolve precipitates and homogenize the microstructure. For duplex grades, the annealing temperature is critical to achieve the target 40–60% phase balance.
- Quenching: Rapid water or forced-air cooling to retain the desired phase composition.
- Pickling and Passivation: Immersion in a mixed acid bath (HNO₃/HF) to remove scale and form a protective chromium oxide passive film.
5.4 Cold Working (if applicable)
For cold-drawn or cold-rolled coil tubes, controlled cold working (5–40% reduction) is applied to enhance mechanical properties while maintaining the target FN. Post-cold-work annealing may be performed at 850–950°C to restore ductility without altering the ferrite content significantly.
5.5 Coiling and Packaging
Finished coil tubes are wound onto precision mandrels, end-sealed, and packaged with desiccant and protective wrapping to prevent surface contamination and corrosion during storage and transportation.
6. Quality Control and Certifications
6.1 In-Process Inspection
- 100% chemical composition analysis of every heat (OES/ICP)
- FN measurement on every production lot (ASTM E1926)
- Dimensional inspection at specified intervals (OD, wall thickness, coil ID)
- Visual surface inspection (no pitting, cracks